The present invention relates to electric fences and more particularly to a monitor for use on electric fences to indicate their correct operation.
Electric fences of the type under discussion are used, for example, for the purpose of keeping domestic or farm animals within an area, or to prevent undesired animals, for example, kangaroos, from entering a property. In its simplest form, an electric fence comprises a single conducting wire strung along a boundary, elevated above the ground by suitable support means, and electrically insulated from the earth. Electrical impulses are supplied to the wire by an energizer which has two terminals, an earth terminal and a positive terminal. The earth terminal is electrically connected to earth and the positive terminal is electrically connected to the wire of the fence.
The energizer produces electrical impulses which are transmitted along the wire, typically at a one second repetition rate. Each impulse has an overall duration of approximately 300 microseconds and a peak potential with respect to the earth of +7000 volts.
An animal which strays into contact with such an electrified wire during transmission of a pulse completes a circuit between the wire and the earth whereby an electric potential difference of 7000 volts is applied across the animal's body during less than 300 microseconds. The effect is an electric shock which is sufficiently unpleasant to cause the animal to remove itself rapidly from the vicinity of the fence, ordinarily within a period shorter than the pulse repetition rate. The potential, pulse width, and repetition frequency are chosen so as not to cause permanent harm to an animal or to a human who, while earthed, touches the wire. Moreover, the shock is sufficiently unpleasant that after relatively few experiences thereof, animals learn not to touch the electrified wire and not too closely approach the fence.
An important advantage of electric fences is their effectiveness in relation to capital cost. Although the physical barrier presented by the fence may be insubstantial, the deterrent to attempts to cross a boundary defined by the barrier is high. The capital cost per unit distance of an electric fence is much lower than the cost of erecting a far more substantial but not necessarily more effective, fence. Moreover, existing wire fences which are not sufficiently strong to protect a boundary may often be made effective at low cost by adaptation so that they can be electrified as described above.
Electric fences frequently extend for miles and may extend for over a hundred miles. Particularly when the fence extends over long distances and/or under very dry ground conditions, the return path provided by the earth may be poor as between the ground on which the animal in contact with the fence is standing and the earth connection of an energizer. Under those circumstances, the shock experienced by an animal in contact with the wire is greatly attenuated and hence much less effective, or even ineffective, as a deterrent. To overcome those difficulties and also to enhance effectivness of such fences for animals of differing heights, it has been practiced to string a number of wires along the boundary, each at a different height above the ground. In that event, each alternate wire has been insulated from ground and is "live", that is to say, energized by repetitive pulses as previously described, while the remaining wires have been connected to the earth terminal of the energizer. The set of alternate wires connected to the earth terminal of the energizer provide a more reliable conductive return path to the earth terminal than is provided by the ground, and an animal touching a pair of adjacent wires then experiences a 7000 volt peak shock between points of contact with the live wire and the earth wire of the pair even under dry ground conditions and at long distances from the energizer.
As electric fences rely for their effectiveness on a correct voltage being provided on the fence, regular monitoring of this voltage is required to ensure the fence is operating correctly. Incorrect operation can be caused by breaks in the fence wire, breakdown of insulators, damage by fallen trees or branches and leakage to earth caused by these falling across the fence wires.
As these fences can extend for large distances the normal method of measuring fence voltage using a voltmeter is laborious and sometimes hazardous due to the high voltages present. Systems have been devised to monitor voltage at the energizer using a transponder system which monitors fence conditions by sending coded analog or digital signals along the fence wire to each transponder. The transponder returns signals to the energizer or controller indicating correct operation of a particular fence section. Such systems are expensive and may not indicate open circuits in the fence sections.
A further known system consists of a fluorescent tube connected between the active and earth wires of the fence and activated by each pulse applied to the fence by the energizer. This system only indicates that a voltage is present in the fence wires but does not indicate whether this voltage is sufficient for correct operation of the fence. Furthermore the light output of the fluorescent tube may be low making it difficult to determine from a distance whether it is lit or not.
The present invention seeks to overcome or at least ameliorate the disadvantages of these prior systems by providing a monitoring device which is simple to operate and install, inexpensive, reliable and gives an accurate indication of the level of fence voltage.